Cellular Respiration
- Sequence glycolysis, pyruvate oxidation, the Krebs cycle, and oxidative phosphorylation
- Trace how electron carriers deliver electrons to the ETC to drive ATP synthesis
- Account for the approximate ATP yield of aerobic respiration from one glucose
Harvesting energy from glucose
Cellular respiration breaks glucose down to release energy and capture it as ATP, the cell’s usable energy currency. It is the near-exact reverse of photosynthesis. Aerobic respiration has four stages: glycolysis (in the cytoplasm), pyruvate oxidation and the Krebs (citric acid) cycle (in the mitochondrial matrix), and oxidative phosphorylation (at the inner mitochondrial membrane). The first stages mostly harvest high-energy electrons onto carriers; the last stage cashes those electrons in for most of the ATP.
Glycolysis, pyruvate oxidation, and the Krebs cycle
Glycolysis splits one 6-carbon glucose into two 3-carbon pyruvate molecules in the cytoplasm, with a net gain of 2 ATP (made by substrate-level phosphorylation) and 2 NADH. It needs no oxygen. If oxygen is present, each pyruvate enters the mitochondrion and undergoes pyruvate oxidation, releasing CO₂ and forming acetyl-CoA plus NADH. Acetyl-CoA then enters the Krebs cycle, which spins twice per glucose, releasing the remaining carbons as CO₂ and loading electrons onto carriers — per glucose the cycle yields about 2 ATP, 6 NADH, and 2 FADH₂.
Oxidative phosphorylation: where most ATP is made
The NADH and FADH₂ collected earlier carry high-energy electrons to the electron transport chain (ETC) on the inner mitochondrial membrane. As electrons pass down the chain, the released energy pumps H⁺ into the intermembrane space, building a gradient. H⁺ then flows back through ATP synthase, driving chemiosmosis to produce the bulk of the cell’s ATP. At the very end, oxygen is the final electron acceptor, combining with electrons and H⁺ to form water. Without O₂ to accept electrons, the whole chain backs up and stops.
Without oxygen: fermentation
If oxygen is absent, the ETC halts and NADH cannot be unloaded, so NAD⁺ runs out and glycolysis would stall. Fermentation solves this: it regenerates NAD⁺ so glycolysis can keep making its small 2 ATP per glucose. In lactic acid fermentation (human muscle, many bacteria), pyruvate is reduced to lactate. In alcoholic fermentation (yeast), pyruvate is converted to ethanol and CO₂. Fermentation makes no additional ATP beyond glycolysis — its whole point is recycling NAD⁺, not producing energy.
Trace the approximate ATP yield from the complete aerobic respiration of one glucose molecule, noting where each contribution comes from.
- 1.Glycolysis produces a net 2 ATP directly (substrate-level phosphorylation) plus 2 NADH.
- 2.Pyruvate oxidation and the Krebs cycle produce 2 ATP directly, and together load many electron carriers (per glucose: about 10 NADH and 2 FADH₂ in total).
- 3.Those carriers deliver electrons to the ETC, and oxidative phosphorylation (chemiosmosis) generates roughly 26–28 ATP — the large majority.
- 4.Add the directly-made ATP (2 + 2 = 4) to the oxidative phosphorylation total (~26–28).
What is the direct role of oxygen in aerobic cellular respiration?
Do not credit the ETC/ATP synthase with making ATP "from oxygen." ATP synthase is powered by the H⁺ gradient (chemiosmosis); oxygen’s job is only to be the final electron acceptor that keeps electrons flowing. Also note glycolysis happens in the cytoplasm, not the mitochondrion.
A muscle cell working hard runs low on oxygen and switches to lactic acid fermentation. What is the main purpose of fermentation?
During which stage of aerobic respiration is the greatest amount of ATP produced?
Keep the tally straight: glycolysis (cytoplasm) → 2 ATP + 2 NADH; pyruvate oxidation + Krebs cycle (matrix) → 2 ATP + more NADH/FADH₂ + CO₂; oxidative phosphorylation (inner membrane) → the bulk of ATP, with O₂ as final electron acceptor forming H₂O. Anaerobically, only glycolysis’s 2 ATP remain.
Answer the 3 checkpoints as you read.
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